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The root habits of the potato (Solanum tuberosum) have been thoroughly studied in many places. At Peru, Nebr., Early Ohio potatoes were grown in mellow loess soil in rows 3 feet apart. The plants were spaced 2 feet apart in the rows. The field was kept clean by shallow hoeing so that the potato roots lying near the surface were not disturbed. Planting was done on April 5, and the roots were examined on May 31.
Early Development.--When the tops were nearly a foot high, the root system was almost entirely in the surface 6 to 8 inches of soil. As many as 55 roots originated from the base of a single plant and ran practically parallel with the surface of the soil (Fig. 108). They varied from a few inches to 2.2 feet in length. None penetrated deeper than 1.5 feet. The soil about the plants was so thoroughly filled with these roots that it was found impossible to represent all of them in one plane. Consequently, the drawing shows but one-half of the entire root system. The close proximity of the roots to the soil surface, some being entirely confined to the first 2 inches, should be taken into account in tillage practice. Deep cultivation would have been very destructive to the roots and would undoubtedly have upset the nicely adjusted balance between root and shoot, resulting in a materially decreased yield.
Fig. 108.--One-half of the root system of an Early Ohio potato plant, 56 days old.
It should be noted that a number of the longer roots showed a distinct tendency to turn rather abruptly downward. This is a marked character in the normal growth habit. The main roots were densely covered with thread-like branches varying in length from a fraction of an inch to 4 inches. These were so numerous that the surface soil was thoroughly occupied by them. Several young tubers had begun to appear.
Mature Root System.--When growth was complete and about one-third of the leaves dead, the plants were again examined. This was on July 8, 94 days after planting. One-half of the mature root system is shown in Fig. 109. The form of the root system was almost identical with that found 5 weeks earlier. Practically the only difference was in its extent. With few exceptions, the roots ran outward and downward in the surface foot and often in the surface 8 inches for distances varying from 6 inches to over 2 feet. They then curved more or less abruptly downward and continued their irregular course to a depth of 2 to 4.7 feet. Some were still distinctly shallow, running their entire course in the surface 2 to 3 inches of soil. The paucity of roots penetrating vertically downward beneath the plant is in striking contrast to the habit of corn and the smaller cereals. All the roots, whether shallow or deep, were freely branched throughout their course, even to their tips. These fine, white, frequently rebranched laterals were usually 0.5 inch to 4 inches long, although some near the surface reached 15 inches.
Fig. 109.--One-half of the root system of a mature potato plant.
Thus, in its early growth, the potato has a distinctly superficial, widely spreading root system. Later the roots turn downward and rather thoroughly occupy the second, third, and part of the fourth foot of soil. The soil volume directly under the plant is often less completely occupied. The individual plants were more variable in respect to number and extent of roots than any of the monocotyledonous crops.
During a following season, the tendency of the roots to turn downward after spreading widely was not so marked, perhaps not more than 30 per cent penetrating far below the 1.5- to 2-foot level. Drier soil both above and below the second foot may have accounted, in part, for this difference. The root ends were much more branched than previously. In all other respects, the root habit was similar.
Root Habit in Relation to Tillage.--The relation of the root habit of potatoes to methods of cultivation is apparent. It is very similar to that of corn. Thorough tillage should be given during the early development of the crop so as to afford the most favorable environment for the growth of the roots and tubers. Each successive cultivation should be shallower than the preceding one and farther away from the plants. Usually, during the final cultivation, enough soil should be worked in towards the vines to give a little ridging. This protects the tubers growing near the surface from sunburn and frost. Careless cultivation or its continuance too late in the season often causes low yields. A single cultivation when the soil is well filled with roots may, in the absence of a rain soon after cultivation, reduce the yield fully one-half. 198
Variations in Root Habit under Different Degrees of Moisture.--Like other cultivated plants, the potato gives a marked response to differences in environment. Bliss's Triumph potato, widely cultivated in Colorado as an early variety, was grown in an unirrigated plot of fine sandy loam soil at Greeley, during 2 successive years. 104 As indicated by the water content of the soil, the second year was one of much greater rainfall. The roots of plants grown in the same field during the 2 years are shown in Fig. 110. The hills were spaced 14 inches apart in rows 3 feet distant. During the drier year, no water was available in the third foot and only a small amount in the surface soil. Scarcity of water promoted an early extensive root development. The exceedingly numerous, long, much-branched laterals thoroughly ramified the dry soil. Owing to hot, dry weather, coupled with little available water, which at certain levels was practically none, the plants did not increase in size between June 23 and the next examination on July 7. They wilted almost daily and regained partial turgidity only during the cool nights. Under these conditions, growth was poor. The roots had scarcely developed beyond the state reached 2 weeks earlier. In fact, the lateral spreading and branching had not changed, except that the branches now extended to the root tips, indicating the cessation of root elongation. The working depth, however, had been increased 6 inches, and the maximum penetration from 23 to 26 inches. Branching was profuse throughout. Laterals occurred at the rate of 15 to 20 per inch and ranged in length from ½ inch to 18 inches. These were thickly rebranched to near their tips.
Fig. 110.--One-half of the root systems of Bliss's Triumph potato grown in the same soil but with different amounts of water. The more extensive root system developed under a favorable water content.
In striking contrast was the extensive root habit in the same soil, when water, although not abundant, was sufficient for continuous growth. The earlier examination showed that the branches, although the same in number as in the previous year, were decidedly shorter. They averaged only 3 inches (maximum 11) as compared with 5 inches (maximum 18). At the later examination, the tops were 17 inches tall as compared with 8 inches for the dry year and lacked only 1 foot of occupying all the space between the 3-foot rows. In spite of the excellent root system, even these plants showed symptoms of drought. Many leaves had died, and others were half dry. Correlating with the better development aboveground, the roots had extended their absorbing area to a working level of 3 feet, and a maximum penetration of 46 inches, was attained. Branching was profuse throughout. The previous yield of 19 bushels per acre was increased to 29 bushels, the tops having made too luxuriant a growth to resist drought, which later caused the crop to dry early and thus reduced the yield.
Root habits of similar crops grown in adjacent irrigated plots are well worth comparison. Here, the plants, late in June of the dry year, had larger tops but much less extensive root systems. At the June examination, root habit in the lightly and fully irrigated plots was identical. These plots had been fertilized at the rate of 5 tons of barnyard manure per acre before the seed bed was prepared. Root habit differed from that in the dry land in a slightly shallower penetration, fewer roots occurring in the second foot of soil, and especially in fewer and shorter branches. At the July examination, when the plants were blooming, the fully irrigated crop was rooted mostly in the surface 16 inches of soil, although a few roots had penetrated an inch into the third foot. The lateral spread was about 2 feet on all sides of the hill as compared with 2.5 feet in the dry land. The lightly watered potatoes had a greater lateral spread than the fully irrigated ones. Moreover, the second foot of soil was occupied much more thoroughly, since more roots took a vertically or nearly vertically downward course. Branching was more profuse; the working level was 8 inches deeper and the maximum penetration 7 inches greater. The largest yield, 303 bushels per acre, was made by the lightly watered plants which had also the most extensive root systems.
Other Investigations on the Potato.--Investigations at other stations indicate that potatoes frequently do not extend so deeply as pictured in Figs. 109 and 110. This may be explained, in part, by the different rooting habits among varieties although soil conditions are often equally important. At Geneva, N. Y., roots of the White Star potato reached a maximum depth of only 1.6 feet. The horizontal roots, however, were traced to a distance of 2.2 to 2.5 feet from the base of the plant. Most of the roots were in the 14 inches of surface Soil. 14 At Fargo, N. D., it has been found that late-maturing varieties root more freely and more deeply than early ones. For example, the Early Ohio variety, 43 days after planting, had its roots confined for the most part to 8 inches of surface soil, although a few extended to a depth of 1.5 feet, and some of the horizontal ones reached a length of 2 feet. At maturity, the roots had penetrated to a depth of 2.5 feet. But the Rural New Yorker No. 2, a late-maturing variety, reached a depth of 3 feet, the lateral roots being well interlaced between the hills, which were 3 feet apart. 203 Investigations in Colorado have shown that, in good soil, the roots will often penetrate 2 feet in depth and extend laterally 2 to 3 feet. 177 Depths of penetration of 4 to 5 feet have been reported for potatoes grown in loose and well-drained soils in Utah. 195
Experiments in which the soil layers, at various depths, were fertilized with sodium nitrate showed clearly that the root growth is markedly affected by the presence of fertilizers. Whenever the roots entered an enriched layer of soil, they branched much more freely, while growth in depth was considerably retarded. In most cases, the growing root system was 6 to 8 inches shorter in the fertilized soil, even in mature plants.
Potatoes have a more superficial root system than many crops such as corn, winter wheat, sugar beets, and most legumes. In its early growth, it is almost entirely confined to 8 inches of surface soil. After extending horizontally to a distance of 1 to 2 feet or more, the roots turn more or less abruptly downward and penetrate the second and third foot of soil. Roots may also occur in the fourth foot. Branching is very profuse throughout the root extent, and at maturity, laterals occur to the root tips. Usually the branches are relatively short but so numerous and well rebranched that the absorbing system is very efficient. There is some evidence which indicates that late-maturing varieties root deeper than early ones. Both depth of penetration and lateral spread, as well as abundance and length of branches, are greatly modified by differences in the water content and fertility of the soil.